Both government and industry are in need of better, lower cost remediation of contaminated land sites. Objectives of the invention include a single composition of fix heavy metals and organics by injection without requiring soil mixing for sites with minimal contamination and for heavily contaminated sites injection of a first composition to precipitate heavy metals and a second composition to fix the precipitate to prevent leaching. Prevention of migration, isolation through impermeable barriers and containment of organics, inorganic metals and salts and radionuclides are desirable. The subject invention covers reduction of chromium from a valence of 6, Cr VI, to a valence of 3, Cr III, to allow precipitation as an hydroxide along with other heavy metals and immobilization in situ of the precipitate to prevent migration. Containment of organic compounds such as PCB, polychlorinated, biphenyls, would be expected with the same treatment. We found in the laboratory that a dual solution treatment allows use of a first solution for reduction of hexavalent chromium to trivalent chromium which then co-precipitates in calcarious type soils, or soils with pH adjusted about 8.5, as an hydroxide along with other heavy metals comprising cadmium, copper, nickel, zinc and lead. For most effective heavy metal co-precipitation we have found a large excess of iron salt over stoichometric amount for chromate reduction is necessary. Further, we have found that a second solution containing approximately from one-half to seven and one-half percent sodium silicate and approximately 0.2 to one percent sodium aluminate when injected or mixed into the contaminated soil following the first solution treatment forms a gel to most effectively contain heavy metals. We would expect that such a gel would also contain most any organic or metallo-organic compounds in soil that is non-saturated with these contaminates. More limited laboratory data indicates that with chromium contaminant at about 30 ppm in soil that a single step solution treatment reduces and precipitates chromium while also reducing soil permeability.
In the absence of chromate in the heavy metal contaminated soils a first solution containing ferric iron salt is used to co-precipitate as hydroxides the heavy metals with the iron. Co-precipitation has been shown in previous work by the inventor to reduce each of the other heavy metals to lower level than would be attained by hydroxide precipitation of one metal alone. Use of soluble treating compositions allows simple injection in the soil to permeate and precipitate heavy metals and use of a second gelling or stabilizing solution to re-saturate the soil to form a gel to reduce the leaching rate or permeability to the point of being almost impermeable.
Actual containment of heavy metals and/or organics in the contaminated soil will be a function of soil permeability or aqueous flow-through rate and concentration of the heavy metals or organics that may be leached out of the gel. Using known equations and data from soil samples after treatment with the iron solution and after formation of the stabilizing gel, calculations indicatate that to replace one volume of the liquid saturating the contaminated soil with an aqueous leaching agent should require over 250 years. This assumes continued stability of the gel structure. At any rate since we would expect the gel structure to be renewable so as an effective remediation method is indicated.
The prior art that we find that is closet to the subject invention is European Patent application, Publication No. 0 352,096 A2 which is application number 89307329.6 filed Jul. 19, 1989 by Lapat Industries. The process in the application is aimed at treating sludge, soils and slurries and requires mixing a reducing agent such as ferrous salt or sodium sulfite with the material being treated to reduce chromate VI to chromate III and addition of sodium silicate primarily to form metasilicates with anions and secondarily to form a silicate shell. In all cases calcium or magnesium oxides are used. Our invention is based on a different chemical approach; that is, to co-precipitate heavy metals in place as hydroxides, at the same time and with the same iron salt used to reduce chromium VI to chromium III. We have found co-precipitation as hydroxides more completely precipitates metals than single hydroxide precipitation. Further, we use only soluble components in our sodium silicate addition in order that we may saturate soils in place to achieve the most economical treatment.